Method and measuring arrangement for measuring paper surface

ABSTRACT

The invention relates to a method and a measuring arrangement for determining gloss of paper or paperboard. The paper or paperboard surface is illuminated with collimated light substantially parallel with the normal of the paper or paperboard surface. An image is produced from the paper or paperboard surface by means of reflected optical radiation. The reflected optical radiation is used for producing an image of the paper or paperboard surface onto the pixels of the camera&#39;s detector surface, each of the pixels imaging an area in the order of micrometers of the paper or paperboard surface. An aperture provided in front of the camera is used for adjusting the incidence angles of the reflected radiation, which affects the measurement of gloss. Gloss of paper or paperboard is measured on the basis of the intensity of the pixels on the camera&#39;s detector surface.

This application is a Continuation of International ApplicationPCT/F100/00411 filed on May 9, 2000, which designated the U.S. and waspublished under PCT Article 21(2) in English, and which is incorporatedherein in its entirety by reference.

FIELD OF THE INVENTION

The solution according to the invention is used for measuring a surfaceproperty of paper or paperboard. The paper or paperboard surface isilluminated and the illuminated paper or paperboard surface is imaged bya camera for measuring.

BACKGROUND OF THE INVENTION

The properties used for describing the surface properties of paper andpaperboard include roughness and gloss which are used e.g. forestimating the printability of paper. The probably most common way ofmeasuring paper roughness is based on airflow measuring methods whichare employed in the apparatuses of Bendtsen, Parker-Prit-Surf-Sheffield,Bekk and Gurley Hill, for example. In these solutions a cylindricalelement is pressed against the paper or paperboard surface with acertain amount of force. A pressure difference with respect to theambient air is caused in the element, and the air flowing through anopening between the edge of the cylindrical element and the paper orpaperboard surface is measured. There are several problems related tothese measurements. The parameter representing roughness is alsoinfluenced by the porosity of paper. The measurement is also integrallydirected at a large surface area, for which reason accurate informationon the detailed structure of the paper surface cannot be obtained. Thisis disadvantageous to the estimation of printability, for example. Afurther disadvantage is an error factor caused by the dependency betweenthe measuring result and the force used for pressing the cylindricalelement against the paper or paperboard surface.

The surface properties of paper and paperboard can also be measuredoptically. In profilometric roughness measurement the paper surface isexamined unidimensionally using a focused ray in the same way as whenreading a CD. Problems are caused by porosity, paper transparency andstrong reflectance, if any, which distort the profile. Unidimensionalityof the measurement is also a disadvantage. Roughness can also bemeasured utilizing light scattering. In the prior art solution the papersurface is conventionally illuminated at an oblique angle (45° to 85°)with respect to the normal of the surface, and the intensitydistribution of radiation from the direction of specular reflection ismeasured to determine roughness. This solution is described in greaterdetail e.g. in Finnish publication no. 56453, which is incorporatedherein by reference. Specular reflection is typically also measured inconnection with the measurement of the gloss. In the standardizedsolution according to the prior art, gloss of paper or paperboard ismeasured by illuminating the paper or paperboard surface at a largeangle (e.g. 75°) with respect to the surface normal and detectingoptical radiation reflected from the surface at the angle of reflection.This solution is described more closely in standard T 480 om-92, whichis incorporated herein by reference. The prior art measuringarrangements measure gloss and roughness integrally from a large areathe size of which is usually dozens of square millimeters. The averagemeasured from a large area does not give accurate information on thestructure of the paper surface and thus e.g. printability cannot beestimated reliably. The large measuring angle further impairs themeasuring accuracy. An additional problem is that the surface roughnessand the surface gloss are mixed with each other and they cannot bedetermined separately with any certainty.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to provide a method and an apparatusimplementing the method to eliminate the above-mentioned problems. Thisis achieved with the solution according to the invention which relatesto a method of determining a surface property of paper or paperboard,the method comprising illuminating the paper or paperboard surface andimaging the illuminated paper or paperboard surface onto the pixels of acamera's detector surface. For measuring at least gloss as the microsurface property, the method according to the invention comprisesilluminating the paper or paperboard surface with collimated lightsubstantially parallel with the normal of the paper or paperboardsurface; controlling the reflected optical radiation with an aperture ofthe desired size before the camera; producing an image of the paper orpaperboard surface onto the pixels of the camera's detector surface bymeans of the reflected optical radiation, each of the pixels imaging anarea in the order of micrometers of the paper or paperboard surface, andmeasuring at least the gloss of paper or paperboard surface on the basisof the intensity of the pixels on the camera's detector surface.

The invention also relates to a measuring arrangement for determining asurface property of paper or paperboard, the measuring arrangementcomprising an optical power source for illuminating the paper orpaperboard surface and a camera for imaging the illuminated paper orpaperboard surface onto the pixels of the camera's detector surface. Themeasuring arrangement according to the invention comprises, formeasuring at least one micro surface property, a collimating opticalblock for collimating the optical radiation emitted from the opticalpower source; a partially permeable mirror for projecting the opticalradiation onto the paper or paperboard surface substantially parallelwith the normal of the paper or paperboard surface; an imaging opticalblock for producing an image of the illuminated paper or paperboardsurface onto the pixels of the camera's detector surface by means of thereflected optical radiation; an aperture of the desired size forcontrolling the reflected optical radiation; the measuring arrangementbeing arranged to measure at least gloss of the paper or paperboardsurface on the basis of the intensity of the pixels on the camera'sdetector surface.

The dependent claims disclose the preferred embodiments of theinvention.

The solution according to the invention is based on illuminating thepaper or paperboard surface with collimated optical radiation from thedirection of the surface normal. An aperture is used for controlling thequality of the micro surface property to be measured. When gloss ismeasured, the aperture is used for adjusting the direction of opticalradiation entering the camera. When roughness is measured, the apertureis used for adjusting the depth of focus of the image formed onto thepixels of the camera's detector surface. In both cases the micro surfaceproperties are measured by means of the intensity received by the pixelson the camera's detector surface.

The measuring arrangement and method according to the invention provideseveral advantages. The invention enables measuring of a surfaceproperty of paper or paperboard from a surface area in the order ofmicrometers, in which case the surface properties can be determined asmicro surface properties. This allows to separate gloss and roughnessreliably from each other. Thanks to this, the printability of paper orpaperboard can be estimated accurately, which results in considerablesavings in the production costs of various printed goods because paperor paperboard processing can be controlled during the manufacture. Inpractice this means that the consumption of ink used in printing, forexample, can be optimized, which allows to guarantee good quality of thetext and pictures.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in greater detail by means of preferredembodiments, with reference to the accompanying drawings, in which

FIG. 1 illustrates a measuring arrangement for measuring micro surfaceproperties of the paper or paperboard surface,

FIG. 2 illustrates a measuring arrangement for measuring gloss of thepaper or paperboard surface,

FIG. 3 illustrates dependency between an optical aperture and opticalradiation reflected from the surface,

FIG. 4 illustrates a measuring arrangement for measuring roughness ofthe paper or paperboard surface,

FIG. 5 illustrates determination of the depth of focus,

FIG. 6A is a top view of a rotatable disc,

FIG. 6B is a perpendicular side view of a rotatable disc,

FIG. 7 illustrates a measuring arrangement for determining micro surfaceproperties of the paper or paperboard surface.

DETAILED DESCRIPTION OF THE INVENTION

The measuring arrangement and method according to the invention are usedfor measuring a micro surface property of the paper or paperboardsurface, which can be utilized in the estimation of printability ofpaper or paperboard. The micro surface property to be measured is gloss.The same measuring arrangement can also be used for measuring roughness.

The simplest way of separating gloss and roughness from each other is tothink that roughness means larger irregularities on the surface to bemeasured than the irregularities affecting gloss. In optical measurementroughness is represented by irregularities which exceed the wavelength(considerably), whereas the irregularities affecting gloss are mainly inthe range of the wavelength. This means that scattered optical radiationconsists of rays reflected in different directions according to thesurface profile and of rays diffracted from the small irregularities onthe surface.

Now we will describe the solution according to the invention moreclosely with reference to FIG. 1. The measuring arrangement comprisespaper or paperboard as a sample 11, an optical power source 14 foremitting optical radiation 15 onto the paper or paperboard 11 surface 10and a collimating optical block 16 for collimating the radiation 15 anda mirror 18 for projecting the collimated radiation 15 onto the sample11 surface substantially parallel with the surface normal of the sample11. The solution according to the invention allows slight deviation inthe direction of the surface normal, e.g. less than 10 degrees. Themirror used in the inventive solution is a partially permeable mirror18, e.g. a semi-permeable mirror. The paper or paperboard 11 ispreferably placed on an even and firm base 12. The measuring arrangementfurther comprises an aperture 20 of the desired size. Reflected opticalradiation 17 consists of specular reflection and part of the scatteredoptical radiation. When gloss is measured, the aperture 20 limits theincidence angle of radiation to the camera 22 (this will be describedmore closely in FIG. 3), i.e. limits the access of scattered radiationonto the camera's 22 detector surface. When roughness is measured, thesize of the aperture 20 determines the depth of focus (described moreclosely in connection with FIG. 5). The measuring arrangement alsopreferably comprises a computer 50 for computer-based image analysis.The collimating optical block 16 and the imaging optical block 13comprise at least one lens. The imaging optical block 13 is dimensionedso that details of the paper or paperboard 11 surface 10 in the order ofmicrometers can be distinguished from the image which is formed on thecamera's 22 detector surface, which allows measurement of micro surfaceproperties from the surface 10. The order of the imaging optical block13, the partially permeable mirror 18 and the aperture 20 of the desiredsize in the route of the reflected optical radiation 17 is not relevantto the invention, i.e. the order may differ from what has been shown inFIG. 1. The camera 22 is e.g. a CCD camera (Charge Coupled Device) whichis known per se and the detector surface of which is a matrix consistingof pixels. The image produced by the camera is preferably processed inthe shades of gray in the computer 50, and therefore the camera may be ablack and white camera.

The optical power source 14 preferably has a wide enough optical band,which allows to avoid any problems caused by the speckle phenomenon. Thespeckle phenomenon is typical particularly in monochromatic opticalsources of radiation. The wider the band of optical radiation, the lessthe speckle images interfere with the measuring according to theinvention. Optical power sources that can be applied in the solutionaccording to the invention include glow filament lamps and gas dischargelamps as well as semi conductive components, such as the xenon lamp orthe LED (Light Emitting Diode). In this application optical radiationrefers to radiation which ranges from ultraviolet to infrared in theoptical band (corresponds approximately to the range of 40 nm to 1 mm).The optical radiation 19 received by the camera 22 forms an image ontothe pixels of its detector surface. The information included in theimage is used for determining a micro surface property of the paper orpaperboard 11 surface 10.

FIG. 2 illustrates a measuring arrangement according to the inventionfor determining micro gloss of the paper 11 or paperboard 11 surface 10.This measuring arrangement comprises the same parts as the measuringarrangement shown in FIG. 1, but the size of the aperture 30 can beadjusted with a control block 23. A further difference is that theradiation emitted by the optical radiation source 14 is projected ontothe paper or paperboard 11 surface using a mirror 35. For this reasonthe collimating optical block is divided into two lenses 16 in FIG. 5.By changing the size of the aperture 30 the incidence angle of theradiation to the camera's detector surface can be adjusted. This allowsto control the amount of scattered radiation to be included in eachmeasurement.

The surface 10 of glossy paper 11 or paperboard 11 has a high specularreflection. The less glossy the paper 11 or paperboard 11 surface 10,the more the surface 10 scatters optical radiation. As illustrated inFIG. 3, optical radiation 31 scattered from the surface 10 passes at anoblique angle with respect to the normal 33 of the surface and does nothit the aperture 30, whereas the optical radiation 32 specular reflectedfrom the surface 10 enters the aperture 30 almost perpendicularly andpasses through the aperture 30 to the camera 22. When a larger aperture30 is used, the ray 31 can also pass to the camera's detector surface. Avery small aperture 30 substantially allows only specular reflectedradiation 32 to the camera's 22 detector surface. Thus the glossier thepaper 11 or paperboard 11 surface 10, the larger the amount of opticalradiation allowed to the camera's 22 detector surface is. The glossierthe paper, the larger the number of pixels is that have receivedspecular reflected rays. The ratio of the scattered radiation (darkpixels or pixels with low intensity) to the specular reflected radiation(bright pixels or pixels with high intensity) is essential to themeasurement of gloss. In the gloss measurement the aperture 30 ispreferably located at the focal point of the optical block 13, in whichcase the aperture can be very small. In that case the camera 22 needs tobe provided with an objective for forming an image of the paper orpaperboard 11 surface onto the camera's 22 detector surface.

When a single pixel distinguishes a surface area unit of e.g. 3micrometers * 3 micrometers from the surface 10, the micro surface areascan be detected one by one according to the solution of the inventionand the gloss of each surface area unit can be determined as microgloss. Instead of one pixel, the surface 10 image can be examined usinga pixel set. The total measurement area of micro gloss may be e.g. 2.6mm * 2 mm and the resolution 3 micrometers. This resolution is the microarea of the surface 10 distinguished/separated by a single pixel or aknown pixel set. Typical pixel sets include a 5×5 pixel matrix or across-shaped pixel set including 5 or 9 pixels, for example.

Images are taken from the same surface 10 area of the paper orpaperboard 11 surface 10 preferably with different aperture sizes usingan adjustable aperture 30. The images are analysed e.g. withcomputer-based image analysis where intensity differences of imagestaken from the same surface 10 area using apertures of different sizesare compared and gloss of the paper 11 or paperboard 11 surface 10 isdetermined on the basis of this comparison. Comparison of intensitydifferences is carried out pixel by pixel, i.e. the intensity obtainedwith a single pixel or a pixel set of an image taken with a certainsized aperture is compared with the intensity value obtained with thesame pixel or a known pixel set of an image produced with adifferent-sized aperture. This allows to determine with the accuracy ofpixel or pixel set how each dot-sized surface area of the surface 10 tobe imaged has reflected optical radiation 15. In other words, the glossof micrometric areas, i.e. micro gloss, can be determined with acomputer-based image analysis program.

In the gloss measurement, gloss can also be estimated on the basis of animage produced with one aperture size only, but at the expense ofaccuracy. In that case gloss is determined from the intensity of asingle pixel or a known pixel set of t least one image produced usingone aperture size. This is preferably performed on the whole imagedarea, which yields information on the gloss of the whole imaged areawith the accuracy of micrometers. When good accuracy is needed in themeasurement of gloss, e.g. when the paper surface is very uniform andglossy, the gloss of such a paper surface has to be determined usingimages produced with more than one aperture size.

FIG. 4 illustrates a measuring arrangement according to the inventionfor determining roughness of the paper or paperboard 11 surface 10. Thismeasuring arrangement comprises the same parts as the arrangement shownin FIG. 1. However, he focal point of the optical block 13 that formsimage on the camera's 22 detector surface can be moved in depth, whichat its simplest is implemented by moving the camera 22 up and down. Afurther difference is that the radiation emitted by the opticalradiation source 14 is projected onto the paper or paperboard 11 surfaceusing a mirror 35. For this reason the collimating optical bloc isdivided into two lenses 16 in FIG. 2. The paper or paperboard 11 ispreferably placed on an even and firm base 12. According to the method,the paper or paperboard 11 surface 10 is illuminated with the opticalpower source 14 via the collimating optical block 16 and partially viathe permeable mirror 18. The collimating optical block 16 collimates theradiation 15 and the semi-permeable mirror 18 projects the radiation 15onto the paper 11 or paperboard 11 surface 10 substantially parallelwith the surface normal of the paper 11 or paperboard 11. Opticalradiation 17 reflected (specular reflection and scattering radiation)from the paper 11 or paperboard 11 surface 10 due to the radiation 15passes via the semi-permeable mirror 18 and the imaging optical block 13into an aperture 20 of the desired size which lets at least part of theoptical radiation 17 reflected from the surface 10 to the camera 22.Thus an image of the surface is formed on the camera's 22 detectorsurface in the same way as in the measurement of micro gloss. Whenroughness is measured, a large aperture provides accurate depthseparation. The size of the aperture can be adjusted according to theneed with a control block 23. In this measurement it is also importantthat the imaging optics allows to distinguish micrometric details fromthe surface 10 image (one pixel corresponds to an area of 3 μm×3 μm, forexample).

Because depth separation is important to the measuring method of microroughness, the method employs as large an aperture as possible. A largeaperture has a low depth of focus, for which reason a large aperture isadvantageous in the measurement of roughness. The average depth of focusis determined e.g. according to the following formulae (factors of theformulae are shown in FIG. 5):${D\quad f} = \frac{\lambda}{\left( {2N\quad A} \right)^{2}}$${N\quad A} = {{n\quad \sin \quad \theta} = \frac{\varphi}{2f}}$${D\quad f} = {\frac{\lambda}{4{\varphi^{2}/4}f^{2}} = \frac{\lambda \quad f^{2}}{\varphi^{2}}}$

The factors of the formulae are: λ=wave length of optical radiation,NA=numerical aperture, n=refractive index of medium, θ=angle at whichoptical radiation enters the objective, φ=diameter of aperture andf=focal distance of lens. As can be seen from the formula, the depth offocus is a function of the square of the diameter's inverse and thus alarge aperture enables accurate measuring of roughness.

In the inventive solution shown in FIG. 4 the optical distance betweenthe camera 22 and the paper or paperboard 11 surface 10 is changedinstead of moving the camera 22 and images are produced at differentoptical distances. Thus the camera's 22 detector surface can be focusedaccording to the height variations of roughness. The optical distancebetween the camera 22 and the paper or paperboard 11 surface 10 can bechanged e.g. by using permeable sheets of different thicknesses in theroute of the optical radiation 17 reflected from the surface 10, whichcan be implemented e.g. with a rotatable disc 60 of FIGS. 6A and 6Bwhich consists of sectors 61 to 63 of different thicknesses. Naturallythe disc 60 has to be made of a material which has a good permeabilityof optical radiation and is optically flawless. The material may be e.g.glass or plastic but other optical materials are also suitable. FIG. 6Ais a top view of the disc 60 and FIG. 6B is a perpendicular side view ofthe disc 60. The images are analysed using e.g. computer-based imageanalysis where changes in the intensity of the pixels of the imagesformed with different depths of focus are compared, which allows to forma height map of the paper 11 or paperboard 11 surface 10. Themeasurement is based on that fact that the intensity of each pixel ismaximal when the image falling on the pixel is focused. The intensity ofnon-focused pixels decreases as the focusing becomes less accurate. Inthe image analysis intensity maxims or image edges are searched forusing an image processing program. This method according to the secondpreferred embodiment of the invention allows to determine the roughnessof paper 11 or paperboard 11 surface 10 even with the accuracy ofmicrometers. However, the accuracy depends on the size of the aperture20.

Now we will describe in greater detail the determination of roughness onthe basis of the images taken. A desired number of images are taken e.g.using standard differences in the depth of focus. To improve quality andto decrease interference, several images can be taken using the samedepth of focus and by calculating the average of the images digitized bythe computer. Then the difference in the intensity of pixels or a groupof pixels is determined between successive images taken using differentdepths of focus. After this, a surface roughness map or another resultdescribing roughness is formed with a computer utilizing the informationthat the intensity of pixels or a pixel set is at its highest at thefocus. It is not necessary to process the whole imaged area formed onthe camera's detector surface in this way, but a representative sampleof the imaged area is sufficient. Such an image processing algorithm hasbeen applied to roughness measurements of metal surfaces in Steurer, J.,Giebel, H., and Altner, W.: Ein lichtmikroskopisches Verfahren zurzweieinhalbdimensionalen Auswertung von Oberflächen, In Proc. 8.DAGM-Symp. Mustererkennung, 1986, Informatik Fachberichte 125, edited byG. Hartmann, pp. 66-70, Springer, Berlin, which is incorporated hereinby reference.

FIG. 7 illustrates a measuring arrangement according to the inventionwhich corresponds to the measuring arrangement shown in FIG. 1, exceptthat the imaging optical block 13 and the aperture 20 of the desiredsize are integrated into the camera 40. The measuring arrangement can beimplemented as described above both in the first and in the secondembodiment of the invention.

In the solution according to the invention both gloss and roughness canbe measured successively with the same measuring arrangement ordetermined even from the same images. Measurements of roughness and/orgloss can be used for controlling the papermaking process of a papermachine to maintain the desired quality of the paper/paperboard to bemanufactured. Information on a surface property or control correctionsdue to surface property measurements can be fed into the controlarrangement of a paper machine either automatically or manually. Thesolution according to the invention is also applicable to continuouson-line measuring of the paper 11 or paperboard 11.

Even though the invention has been described with reference to theembodiment according to the accompanying drawings, it is obvious thatthe invention is not restricted thereto, but may be modified in severalways within the scope of the inventive concept disclosed in the appendedclaims.

What is claimed is:
 1. A method of determining a surface property ofpaper or paperboard, the method comprising illuminating the paper orpaperboard surface with collimated light substantially parallel with thenormal of the paper or paperboard surface; imaging the illuminated paperor paperboard surface onto the pixels of a camera's detector surface;controlling the reflected optical radiation with an aperture of thedesired size before the camera; producing an image of the paper orpaperboard surface onto the pixels of the camera's detector surface bymeans of the reflected optical radiation, each of the pixels imaging asurface area in the order of micrometers of the paper or paperboard;focusing the image of the paper or paperboard surface in depth atdifferent distances by changing the optical distance between the cameraand the paper or paperboard surface and measuring at least the gloss ofthe paper or paperboard surface at different optical distances on thebasis of the intensity of the pixels on the camera's detector surface.2. A method according to claim 1, further comprising measuring roughnessof paper or paperboard also on the basis of the intensity of the pixelson the camera's detector surface.
 3. A method according to claim 2,further comprising determining by the aperture the depth of focus ofimaging in the measurement of roughness.
 4. A method according to claim2, wherein the measurement of roughness comprises the following steps:producing images with the camera using different depths of focus;determining a pixel-specific or a pixel set-specific difference in theintensity between successive images produced using different depths offocus; determining surface height variations utilizing the informationthat the intensity of a pixel or a pixel set is at its highest at thefocus, and determining the roughness of paper or paperboard (11) fromthe surface height variations.
 5. A method according to claim 4, furthercomprising producing several images using the same depth of focus andthe average of digitized images is calculated to decrease interference.6. A method according to claim 4, further comprising forming a heightmap illustrating roughness on the paper or paperboard surface by meansof the images produced.
 7. A method according to claim 1, furthercomprising using the aperture for limiting the incidence angles of thereflected optical radiation to the camera's detector surface formeasuring gloss.
 8. A method according to claim 1, further comprisingmeasuring gloss from the paper or paperboard surface by means ofspecular reflected and scattered radiation by determining the ratio ofthe pixels that have received specular reflected rays to the otherpixels, the intensity of the specular reflected radiation being higherthan the intensity of the scattered radiation.
 9. A method according toclaim 1, wherein the aperture is adjustable.
 10. A method according toclaim 1, further comprising projecting illumination onto the paper orpaperboard surface through a partially permeable mirror.
 11. A methodaccording to claim 1, further comprising comparing the images producedwith one another in computer-based image analysis.
 12. A methodaccording to claim 1, further comprising determining the printability ofpaper or paperboard at least for the gloss measurement.
 13. A methodaccording to claim 1, further comprising measuring paper or paperboardon-line.
 14. A measuring arrangement for determining a surface propertyof paper or paperboard, the measuring arrangement comprising an opticalpower source for illuminating the paper or paperboard surface and acamera for producing an image of the illuminated paper or paperboardsurface onto the pixels of the camera's detector surface, wherein themeasuring arrangement according to the invention comprises: acollimating optical block for collimating the optical radiation; apartially permeable mirror for projecting the optical radiation onto thepaper or paperboard surface substantially parallel with the normal ofthe paper or paperboard surface; an imaging optical block for producingan image of the illuminated paper or paperboard surface onto the pixelsof the camera's detector surface by means of the reflected opticalradiation; and an aperture of the desired size for controlling thereflected optical radiation; and the measuring arrangement is arrangedto change the optical distance between the camera and the paper orpaperboard surface to change the focal point on the paper or paperboardsurface, and the measuring arrangement is arranged to measure at leastgloss of the paper or paperboard surface at different optical distanceson the basis of the intensity of the pixels on the camera's detectorsurface.
 15. A measuring arrangement according to claim 14, wherein themeasuring arrangement is also arranged to measure roughness of paper orpaperboard on the basis of the intensity of the pixels on the camera'sdetector surface.
 16. A measuring arrangement according to claim 15,wherein the aperture is arranged to control the reflected opticalradiation by limiting the depth of focus of imaging in the desiredmanner when roughness is measured.
 17. A measuring arrangement accordingto claim 15, wherein the measuring arrangement is arranged to produceimages with different depths of focuses; determine a pixel-specific or apixel set-specific difference in the intensity between successive imagesproduced with different depths of focus; determine surface heightvariations utilizing the information that the intensity of a pixel or apixel set is at its highest at the focus, and determine roughness ofpaper or paperboard from the surface height variations.
 18. A measuringarrangement according to claim 17, wherein the measuring arrangement isarranged to produce several images with the same depth of focus and tocalculate the average of the digitized images to decrease interference.19. A measuring arrangement according to claim 17, wherein a computer(50) is arranged to provide a height map on the images.
 20. A measuringarrangement according to claim 14, wherein the aperture is arranged tocontrol the reflected optical radiation by limiting the incidence anglesof the reflected optical radiation to the camera's detector surface toimprove the gloss measurement.
 21. A measuring arrangement according toclaim 14, wherein the measuring arrangement comprises a control blockfor adjusting the aperture.
 22. A measuring arrangement according toclaim 14, wherein the measuring arrangement is arranged to measure glossfrom the ratio of the specular reflected radiation to the scatteredradiation by determining the number of pixels that have receivedspecular reflected rays in relation to the number of other pixels, theintensity of the specular reflected radiation being higher than theintensity of the scattered radiation.
 23. A measuring arrangementaccording to claim 14, wherein the camera is a CCD camera.
 24. Ameasuring arrangement according to claim 14, wherein the measuringarrangement comprises an image processing device which is arranged touse an image analysis program to determine a surface property.
 25. Ameasuring arrangement according to claim 14, wherein the measuringarrangement is arranged to determine the printability of paper orpaperboard by means of at least one micro surface property.
 26. Ameasuring arrangement according to claim 14, wherein the measuringarrangement is arranged to measure paper or paperboard on-line.